Microwave oven having tm and te modes

ABSTRACT

A counter-top microwave oven comprises an upstanding box-like metal wall structure defining a heating cavity, a microwave generator, transmission means including a probe antenna for radiating the microwaves into the heating cavity from the top thereof, and tuning means disposed in the heating cavity adjacent to the top thereof, the cavity being so dimensioned and the antenna and tuning means being so positioned as to excite in the cavity a predetermined low impedance transverse magnetic field mode and a predetermined high impedance transverse electric field mode having a low impedance region in the portion of the heating cavity in which food is normally placed. Two alternative positions of the tuning means for establishing the preferred field patterns are disclosed.

United States Patent [191 Staats et a1.

[ MICROWAVE OVEN HAVING TM AND TE MODES [75] inventors: James E. Staats;Louis H.

Fitzmayer, both of Louisville, Ky.

[73] Assignee: General Electric Company,

Louisville, Ky.

22 Filed: Jan. 4, 1974 21 Appl.No.:430,731

Primary Examiner-J. V. Truhe Assistant ExaminerHugh D. .laeger Attorney,Agent, or Firm-Prangley, Dithmar, Vogel, Sandler & Stotland [1113,843,862 [451 Oct. 22, 1974 5 7 ABSTRACT A counter-top microwave ovencomprises an upstanding box-like metal wall structure defining a heatingcavity, a microwave generator, transmission means including a probeantenna for radiating the microwaves into the heating cavity from thetop thereof, and tuning means disposed in the heating cavity adjacent tothe top thereof, the cavity being so dimensioned and the antenna andtuning means being so positioned as to excite in the cavity apredetermined low impedance transverse magnetic field mode and apredetermined high impedance transverse electric field mode having a lowimpedance region in the portion of the heating cavity in which food isnormally placed. Two alternative positions of the tuning means forestablishing the preferred field patterns are disclosed.

PATENTEUncr 22 mm FIG./

1 MICROWAVE OVEN HAVING TM AND TE MODES BACKGROUND OF THE INVENTION Thepresent invention relates to electronic heating apparatus of the typecommonly referred to as a microwave oven, and specifically to amicrowave oven of the countertop variety which has a heating cavity withdimensions generally comparable to a wavelength of the microwave energyused. More particularly, the present invention is an improvement of theinvention disclosed in our copending US. application Ser. No. 430,730,filed Jan 4, 1974, entitled MICROWAVE OVEN WITH PREFERRED MODES, andassigned to the assignee of the present invention.

In that copending application, there were disclosed a number ofdifferent forms of a microwave oven heating enclosure, the dimensionsand antenna location of which provided for the excitation in the heatingcavity of two complementary transverse electric (TE) field modes,thereby resulting in a heating pattern which afforded fairly uniformheating of a wide variety of low impedance food loads.

As an alternative means for achieving composite field patterns havingimproved heating characteristics in microwave ovens, attemps have beenmade to utilize a low impedance transverse magnetic (TM) wave to improvethe microwave heating of low impedance food by reducing the voltagestanding wave ratio in the heating cavity. Results of these attemptshave not been entirely satisfactory because a region in the center ofthe heating cavity did not afford adequate heating. One solution to thisproblem which has given good results is the use of a near field devicedisposed beneath the food load to heat food in the central region of theheating cavity. However, this near field device requires the use of adielectric shelf for supporting the food load.

SUMMARY OF THE INVENTION It is a general object of the present inventionto provide an electronic heating apparatus characterized by a heatingcavity having excited therein a predetermined transverse magnetic fieldmode and a predetermined transverse electric field mode, resulting in acomposite field pattern which provides uniform heating of low impedancefoods throughout the heating cavity.

Another object of this invention is to provide an electronic heatingapparatus of the character described, which includes coupling meanswhereby the TM and TE modes may both be excited from the same antenna.

An important object of the present invention is to provide an electronicheating apparatus for heating a low impedance body, the apparatuscomprising a metal enclosure in the form of a rectangular parallelepipedincluding a top wall, a bottom wall, a front wall, a rear wall and twoopposed side walls defining a heating cavity for receiving therein abody to be heated, source means including a probe antenna electricallycoupled to the heating cavity along the top wall for transmittingmicrowave energy of a predetermined wavelength A into the heating cavityto excite therein two predetermined electromagnetic field modes,'theheating cavity having an electrical width substantially equal to 1.2 Aand an electrical height substantially equal to 0.633 A and anelectrical depth substantially equal to 1.1 A, and tuning means disposedin the heating cavity adjacent to the top thereof, one of thepredetermined electromagnetic field modes being a relatively lowimpedance transverse magnetic field mode, the other of the predeterminedelectromagnetic field modes being a relatively high impedance transverseelectric field mode having a relatively low impedance region generallycentrally of the heating cavity, whereby there is established in theheating cavity an electromagnetic field pattern providing unifonnheating of an associated low impedance body.

Further features of the invention pertain to the particular arrangementof the parts of the electronic heating apparatus whereby theabove-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation,together with further objects and advantages thereof, will best beunderstood by reference to the following specification taken inconnection with the accompanying drawings.

BREIF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front perspective view ofa microwave oven including a heating enclosure constructed in accordancewith and embodying the features of the present invention;

FIG. 2 is a diagrammatic transparent perspective view of a first form ofthe heating cavity of the electronic heating apparatus of FIG. 1 forproducing transverse magnetic and transverse electric field modes; and

FIG. 3 is a view similar to FIG. 2 showing a second form of the heatingcavity of the electronic heating apparatus of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in particular toFIGS. 1 and 2 of the drawings, there is illustrated a microwave oven,generally designated by the numeral 10, constructed in accordance withand embodying the features of the present invention. The microwave oven10 is adapted for placement on top of a table or counter and is housedin a cabinet which includes an upstanding front panel 11, a rear wall12, a top wall 13, a bottom wall 14 and a pair of opposed side walls 16and 17. Mounted within the oven cabinet alongside the front panel 1 1 isa heating enclosure, generally designated by the numeral 20, including atop wall 21, a bottom wall 22, a rear wall 23 and a pair of opposed sidewalls 24 and 26, the front of the heating enclosure 20 being closed by adoor 15 which, in the closed position thereof, forms the front wall ofthe heating enclosure 20, which enclosure is generally in the form of arectangular parallelepiped.

Thus, the heating enclosure 20 serves to separate the interior of theoven cabinet into a heating cavity defined within the heating enclosure20 for receiving therein and heating a food load 18, and a machinerycompartment 25 between the heating enclosure 20 and the oven cabinetwalls l2, l3, l4 and 17. Mounted in the machinery compartment 25 is amagnetron 27, a power supply 28 and a coaxial microwave transmissionline (not shown) which transmits microwave energy from the magnetron 27to the interior of the heating enclosure 20.

The magnetron 27, power supply '28 and transmission line may be of anysuitable type, but preferably they are substantially identical inconstruction and operation to the magnetron, power supplyand'transmission line disclosed in the aforementioned copending US.application Ser. No. 430,730, whereby .there is here described in detailonly so much of the structure of the microwave oven as is necessary toan understanding of the present invention.

The coaxial transmission line from the magnetron 27 hasan outerconductor which is designated by the nuv meral 29 in FIG. 2, andterminates at the top wall 21 of the heating enclosure 20, the innerconductor of the transmission line terminating at a probe antenna 30,

I which projects vertically downwardly into the heating of low impedancefood loads in various positions in the heating cavity. Preferably, theprobeantenna 30 and the tuning probe 35 are so dimensionedas to provideequal heating to a food load placed either in the back or the front ofthe heating cavity. 7

Referring now toFlG. 3 of the drawings, there is illustrated analternative arrangement of the heating enclosure 20, which issubstantially identicaltothe arrangement illustrated'in FIG. 2, exceptthat there is provided a tuning probe 45 extending verticallydownwardlyinto the heating cavity from the top wall 21 at a location thereonsubstantially midway between the side walls 24 and 26 and spacedapproximately 2/4 from the front wall or oven door 15. The sameelectromagnetic tield modes are excited inboth thearrangements of FIGS,2'and 3, the only difference being slight adjustments in the compositeheating patterns.

From the foregoing, it can be seen that there has been provided animproved electronic oven which affords uniform heating of a wide varietyof low impedance food loads by the use of TM and TE field modes v in theheating cavity.

wherein x corresponds to the width of the heating cav ity, y correspondsto the height of the heating cavity and 2 corresponds to the depthv ofthe heating cavity. 30

normally placed in the-heating cavity.

More particularly, there-has been provided an electronic heatingapparatusfwhich includes, a heating cavf ity 'sojdimen'sioned and a.probe antenna and tuning member sopositioned adjacent to the top of theheating cavity'that there isproduced in the heating cavity I apredetermined low impedance TM field mode and a predetermined highimpedance TE field mode havinga low impedance regionin the positionwhere food is There hasv also been provided'an electronic apparatuswhich utilizes both TM'andTE field modes,

excite the TM field mode.

without necessitating the use of a near While there have beend'escribedwhat are at present considered to be the preferred embodiments oftheinvention, it will be understood that'various modificaimpedance, thedimerisiohD of the'heating cavity in the direction'of power flow shouldbe' D" ens-six" It has been found that when the heating enclosure 20has'dimensions according to these relationships, such antenna 30 ispositioned as illustrated in FIG. 2 substa'ntially midway between theside walls 24-and 26 and approximately z/4 fromthe rearwall,23,,thereare pro duced in the heating cavity the TMmM and the TEelectromagnetictield modes, wherein the numeral sub scripts represent,respectively, the field variations in the 'x, y and 2 directions, andthe letter subscript indicatesthe direction of power flow. Y

Aconductive tuning rod or probe 35 is disposed substantially centrallyof the top wall 21*of the heating entionsmay be made therein, and it-isintended to cover in 'thefappendedclaims all such modifications as fallwithin the true spirit and 'scope of the invention.

Whatisclaimedis: r 1. Electronic heating apparatus for heating a low impedancebody, said apparatus comprising a metal en closure inth'e form ofa rectangular parallelepiped including a topwall, a bottom wall,'a'front wall, a rear wall and 'two'oppose'd side walls jdefining aheating ca'v ity for receiving therein abody to beheated, source meansincluding a probe antenna electrically coupled to said heatingcavityalong said top wall "for transmitting microwave energy ofta-predeterminedwavelength closure 20 and extends vertically downwardlyinto the y 1 arelatively high impedance transverse, electric field.

A into said heating cavity to excite therein two prede# terrninedelectromagnetic field modes, said heating cavity havingan electricalwidth substantiallyequal to 1. 2 )t and an electrical heightsubstantially equal to 0.633 A and an electrical depth substantiallyequal-to l.l.)t,'and tun'ing'rneans; disposed in said heating cavityadjacent tothe top thereof, oneof said predetermined electromagneticfield modes beinga relatively low impedance transverse magneticfieldmode, the'jother' of said predetermined electromagnetic fieldmodejsbeing mode havinga relativelylow impedance region, generally,centrallyof said. heating cavity, whereby there is established insaidheatingcavity an electromagnetic field pattern providing uniformheating of an associated lowimpedanee body. J I

heating field device to 4. The electronic heating apparatus set forth inclaim 1, wherein said antenna is disposed substantially midway betweensaid side walls and is spaced from said rear wall an electrical distanceone-fourth the depth of said heating cavity, said tuning means beingdisposed substantially midway between said side walls and being spacedfrom said front wall an electrical distance onefourth of the depth ofsaid heating cavity.

5. The electronic heating apparatus set forth in claim 1, wherein A isapproximately equal to 12.9 inches.

1. Electronic heating apparatus for heating a low impedance body, saidapparatus comprising a metal enclosure in the form of a rectangularparallelepiped including a top wall, a bottom wall, a front wall, a rearwall and two opposed side walls defining a heating cavity for receivingtherein a body to be heated, source means including a probe antennaelectrically coupled to said heating cavity along said top wall fortransmitting microwave energy of a predetermined wavelength lambda intosaid heating cavity to excite therein two predetermined electromagneticfield modes, said heating cavity having an electrical widthsubstantially equal to 1.2 lambda and an electrical height substantiallyequal to 0.633 lambda and an electrical depth substantially equal to 1.1lambda , and tuning means disposed in said heating cavity adjacent tothe top thereof, one of said predetermined electromagnetic field modesbeing a relatively low impedance transverse magnetic field mode, theother of said predetermined electromagnetic field modes being arelatively high impedance transverse electric field mode having arelatively low impedance region generally centrally of said heatingcavity, whereby there is established in said heating cavity anelectromagnetic field pattern providing uniform heating of an associatedlow impedance body.
 2. The electronic heating apparatus set forth inclaim 1, wherein said antenna is disposed substantially midway betweensaid side walls and is spaced from said rear wall an electrical distanceone-fourth the depth of said heating cavity.
 3. The electronic heatingapparatus set forth in claim 1, wherein said antenna is disposedsubstantially mdiway between said side walls and is spaced from saidrear wall an electrical distance one-fourth the depth of said heatingcavity, said tuning means being disposed centrally of said top wall. 4.The electronic heating apparatus set forth in claim 1, wherein saidantenna is disposed substantially midway between said side walls and isspaced from said rear wall an electrical distance one-fourth the depthof said heating cavity, said tuning means being disposed substantiallymidway between said side walls and being spaced from said front wall anelectrical distance one-fourth of the depth of said heating cavity. 5.The electronic heating apparatus set forth in claim 1, wherein lambda isapproximately equal to 12.9 inches.